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Nor-QD (Norethindrone) - Description and Clinical Pharmacology

 
 



PHYSICIAN LABELING
Patients should be counseled that oral contraceptives do not protect against transmission of HIV (AIDS) and other sexually transmitted diseases (STDs) such as Chlamydia, genital herpes, genital warts, gonorrhea, hepatitis B, and syphilis.

DESCRIPTION

Each yellow Nor-QD® tablet provides a continuous oral contraceptive regimen of 0.35 mg norethindrone daily, and the inactive ingredients include D&C Yellow No. 10, FD&C Yellow No. 6, lactose, magnesium stearate, povidone, and starch.

The chemical name for norethindrone is 17-Hydroxy-19-Nor-17α-pregn-4-en-20-yn-3-one. The structural formula follows:

norethindrone

norethindrone

Therapeutic class = oral contraceptive.

CLINICAL PHARMACOLOGY

1. Mode of Action. Nor-QD progestin-only oral contraceptives prevent conception by suppressing ovulation in approximately half of users, thickening the cervical mucus to inhibit sperm penetration, lowering the mid-cycle LH and FSH peaks, slowing the movement of the ovum through the fallopian tubes, and altering the endometrium.

2. Pharmacokinetics.

Absorption: Norethindrone is rapidly absorbed with maximum plasma concentrations occurring within 1 to 2 hours after Nor-QD administration (see Table 1). Norethindrone appears to be completely absorbed following oral administration; however, it is subject to first pass metabolism resulting in an absolute bioavailability of approximately 65%.

Peak plasma concentrations occur approximately 1 hour after administration (mean Tmax 1.2 hours). The mean (SD) Cmax was 4816.8 (1532.6) pg/mL and generally occurred within 1 hour (mean) of tablet administration, ranging from 0.5 to 2 hours. The mean (SD) Cavg was 885 (250) pg/mL, however, the mean concentration at 24 hrs was 130 (47) pg/mL.

Table 1 provides summary statistics of the pharmacokinetic parameters associated with single dose Nor-QD administration.

Table 1: Mean ± SD Pharmacokinetic Parameters Following Single Dose Administration of Nor-QD in 12 Healthy Female Subjects Under Fasting Conditions
 Pharmacokinetic Parameter  Norethindrone 0.35 mg
 Tmax (hr)  1.2 ± 0.5
 Cmax (pg/mL)  4817 ± 1533
 AUC(0-48) (pg·h/mL)  21233 ± 6002
 t½ (h)  7.7 ± 0.5

The food effect on the rate and extent of norethindrone absorption after Nor-QD administration has not been evaluated.

Distribution: Following oral administration, norethindrone is 36% bound to sex hormone-binding globulin (SHBG) and 61% bound to albumin. Volume of distribution of norethindrone is approximately 4 L/kg.

Metabolism: Norethindrone undergoes extensive biotransformation, primarily via reduction, followed by sulfate and glucuronide conjugation; less than 5% of a norethindrone dose is excreted unchanged; greater than 50% and 20-40% of a dose is excreted in urine and feces, respectively. The majority of metabolites in the circulation are sulfate, with glucuronides accounting for most of the urinary metabolites.

Excretion: Plasma clearance rate for norethindrone has been estimated to be approximately 600 L/day. Norethindrone is excreted in both urine and feces, primarily as metabolites. The mean terminal elimination half-life of norethindrone following single dose administration of Nor-QD is approximately 8 hours.

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